1. Field of the Invention
The present invention deals with the field of piston ventilators for patients in need of respiratory assistance. More particularly, the invention is concerned with a control circuit for a piston ventilator that corrects for errors in the gas pressure delivered to the patient compared to a set point pressure by taking into account the error magnitude, the integration of the error over time and gas leakage in the system. The control circuit also determines the set point pressure as one of a plurality of sequential set point pressures on a pressure trajectory during patient inspiration.
2. Description of the Prior Art
The prior art discloses control devices for piston ventilators in which analog signals representative of motor current and desired current are compared to generate an analog error signal. The error signal is used to drive the piston motor in order to minimize the error. In the prior art, a brushed DC motor was used. The relationship between current and torque, i.e., the torque constant for a DC motor, was used to generate specific pressures. Specifically, torque equals force times distance, therefore, for a given torque on a piston, a particular pressure could be generated.
These prior art control schemes present a number of problems. For example, the torque constant varies with temperature. As a result, the error is never eliminated in a controlled fashion. The prior art uses current control which is a function of how frictionless the piston is. Moreover, the signals delivered to the piston motor do not take into account gas leaks in the system. Also, the prior art analog control schemes do not provide the desired level of precision and flexibility in operation needed for medical applications.